Major oil-producing countries

As mentioned above, petroleum resources are not distributed evenly around the world. Indeed, according to estimates published for 2015 by the U.S. Department of Energy, as few as 15 countries account for more than 75 percent of the world’s oil production and hold roughly 93 percent of its reserves. Significantly, those countries are projected to have a correspondingly large percentage of the world’s remaining undiscovered oil resources, which are estimated by the extrapolation of known production and reserve data into untested sediments of similar geology.

Saudi Arabia has the second largest proven oil reserves in the world—some 268 billion barrels, approximately 16 percent of the world’s proven reserves—not to mention significant potential for additional discoveries. The discovery that transformed Saudi Arabia into a leading oil country was the Al-Ghawār oil field. Discovered in 1948 and put into production in 1951, this field has proved to be the world’s largest, generating an estimated 55 billion barrels after 60 years of production. Saudi officials estimate that this field contains more than 120 billion barrels in recoverable reserves, if waterflooding (that is, water injection that forces oil from the oil reservoir) is considered. Another important discovery was the Saffāniyyah offshore field in the Persian Gulf in 1951. It is the third largest oil field in the world and the largest offshore. Saudi Arabia has eight other supergiant oil fields. Saudi fields, as well as many other Middle Eastern fields, are located in the great Arabian-Iranian basin.

Major oil fields of the Arabian-Iranian basin region.Encyclopædia Britannica, Inc.

The Middle Eastern countries of Iraq, Kuwait, and Iran are each estimated to have had an original oil endowment in excess of 100 billion barrels. Together they account for more than 23 percent of all proven reserves in the world. These countries have a number of supergiant fields, all of which are located in the Arabian-Iranian basin, including Kuwait’s field at Al-Burqān, which was discovered in 1938. Al-Burqān is the world’s second largest oil field, having originally contained 75 billion barrels of recoverable oil. Iraq possesses a significant potential for additional oil discoveries, primarily in its southwestern geographic region, where an estimated 45–100 billion barrels of crude oil are thought to reside. This resource has been slow to develop, because of the country’s involvement since 1980 in major wars and subsequent civil unrest.

Russia is thought to possess the best potential for new discoveries. It has significant proven reserves—some 80 billion barrels, approximately 6 percent of the world total—and is one the world’s leading petroleum producers. Russian oil is derived from many sedimentary basins within the vast country, and two supergiant oil fields, Samotlor and Romashkino, were discovered in 1964 and 1949 respectively. Production from these mature fields is on the decline, however, so that total Russian oil output is maintained by production at new fields. The best prospects for new Russian discoveries appear to exist in difficult and expensive frontier areas such as Sakhalin Island.

The Tengiz field on the northeast border of the Caspian Sea is a supergiant with up to 9 billion barrels recoverable reserves. It was originally discovered in 1979; however, it was not actively developed until the American oil company Chevron gained equity in the region in 1993. Operating equipment and producing oil in this field are extremely complex because of the oil’s high levels of hydrogen sulfidegas, extremely high well pressure, and large volumes of natural gas.

Kazakhstan’s Kashagan field in the northern Caspian Sea was discovered in 2000. It was the largest and newest conventional field discovered since the finding of Alaska’s Prudhoe Bay field in 1968. Kashagan is estimated to have already produced 7 billion to 9 billion barrels out of its proven 30-billion-barrel reserves.

Sub-Saharan Africa

Sub-Saharan Africa, primarily West Africa, holds a rich resource base with multiple supergiant and giant fields. Beginning to the north, Ghana boasts the most recent potential supergiant, the Jubilee field, with potential reserves of 2 billion barrels. It was discovered in 2007 and produced more than 110,000 barrels per day by 2011. However, the majority of sub-Saharan African recoverable reserves and supergiant or giant fields are in Nigeria, Angola, Equatorial Guinea, and Gabon.

Nigeria’s Niger delta harboured the country’s first commercial oil discovery, the Oloibiri oil field, which is now referred to as Oil Mining Lease (OML) 29. The Niger delta province spans from onshore to deepwater offshore and holds upward of 37.4 billion barrels of oil and 193 trillion cubic feet of gas reserves. Several reservoirs make up the total play, with the giant Agbami light sweet crude field having over 1 billion barrels of recoverable reserves. Agbami was discovered in 1998 and began to produce some 10 years later. Outside the Niger delta is the giant deepwater oil field Bonga, or OML 118, discovered in 1996 southwest of the Niger delta. With recoverable reserves of 600 million barrels, OML 118 began to produce in 2005.

Angola and its Cabinda province have recoverable reserves totaling more than 9.5 billion barrels of oil and 10.8 trillion cubic feet of natural gas. Block 15 is the largest producing deepwater block in Angola. The offshore petroleum development zone is located in the Congo basin and has estimated total recoverable hydrocarbon reserves of 5 billion barrels. Discovered by ExxonMobilaffiliate Esso Exploration Angola in 1998, the giant Kizomba field with over 2 billion barrels of recoverable reserves launched Angola’s commercial production rise and the country’s membership in OPEC in 2007. The development of the Kizomba field was a phased-in process, with production beginning in 2004 and full development occurring in 2008. Angola’s Block 17 includes the Dalia and Pazflor fields. Dalia was first discovered in 1997. It began production in 2006 and has estimated recoverable reserves of 1 billion barrels. Pazflor field, discovered in 2000 and located northeast of Dalia, is estimated by the operator, Total, to contain recoverable reserves of 590 million barrels. The field first began to produce in 2011.

Equatorial Guinea has an estimated 1.1 billion barrels of recoverable reserves and boasts the first deepwater field brought online in West Africa. The giant Zafiro field was discovered in 1995 by ExxonMobil and Ocean Energy. It is located northwest of Bioko island, and it contained the bulk of the country’s recoverable reserves. Zafiro began production using a floating production storage and offloading vessel in 1996. Equatorial Guinea’s major hydrocarbon contribution, however, is its natural gas resources. The Alba field is estimated to have up to 4.4 trillion cubic feet of reserves or an equivalent 759 million barrels of oil. This enormous supply allowed government officials to justify significant infrastructure development on Bioko island for exporting liquefied natural gas and oil.

Gabon is West Africa’s second largest reserves holder with 2 billion barrels of recoverable reserves. The giant Rabi-Kounga field was discovered in 1985 and began production in 1989. Originally, Rabi-Kounga was estimated to have 440 million barrels of reserves, but this was increased in 1993 to 850 million barrels following a reappraisal, the creation of additional facilities, and infill drilling by the Shell petroleum company. By the early 21st century, however, only a fraction of this amount remained for further production.

North America has many sedimentary basins. Basins in the United States have been intensively explored, and their oil resources developed. More than 33,000 oil fields have been found, but there are only two supergiants (Prudhoe Bay, in the North Slope region of Alaska, and East Texas). Cumulatively, the United States has produced more oil than any other country. Its proven oil reserves amount to some 40 billion barrels, representing approximately 2 percent of the world total, but the country is still considered to have a significant remaining undiscovered oil resource. Prudhoe Bay, which accounted for approximately 17 percent of U.S. oil production during the mid-1980s, is in decline. This situation, coupled with declining oil production in the conterminous U.S., contributed to a significant drop in domestic oil output through the end of the 20th century. In the early 21st century, however, advancements in unconventional oil recovery resulted in skyrocketing production, and by 2015 the U.S had become the world’s leading petroleum-producing country.

Sedimentary basins and major oil and gas fields of North America.Encyclopædia Britannica, Inc.

Mexico has more than 10 billion barrels of proven oil reserves and is one of the top 10 oil producers in the world. However, its principal supergiant oil field (Cantarell, offshore of Campeche state), which is one of the largest conventional oil fields discovered in the Western Hemisphere, peaked at more than 2 million barrels per day in 2003, making it difficult to sustain current production levels well into the 21st century. Mexico’s potential supergiant, Chicontepec, which contains roughly 40 percent of the country’s reserves, is estimated to hold 17 billion barrels of oil equivalent. However, most of the oil is extra-heavy crude, and this circumstance has hampered development.

Canada has less than 10 billion barrels of proven reserves of conventional liquid oil, but huge deposits of oil sands in the Athabasca region of Alberta in western Canada bring the country’s total proven oil reserves to more than 170 billion barrels, behind only oil giants Venezuela and Saudi Arabia. Canada’s largest oil field is Hibernia, discovered in the Jeanne d’Arc basin off Newfoundland in 1979. This giant field began producing in 1997 and was soon joined by two other fields, Terra Nova (first production 2002) and White Rose (first production 2005).

Venezuela is the largest oil exporter in the Western Hemisphere and has long been an important country in the world oil market. With approximately 298 billion barrels of proven oil reserves, it has the world’s largest oil endowment. Most of the estimated 500 billion barrels of potential reserves, however, are in the form of extra-heavy oil and bitumen deposits located in the Orinoco belt in the central part of the country, which have not been exploited to a large extent. The country’s most important producing field is the Bolivar Coastal field. Discovered in 1917, this complex of large and small reservoirs is found in the Maracaibo basin in the west. These mature fields have produced over 70 percent of the estimated recoverable reserves, but they are declining in production.

Since the late 20th century, Brazil has emerged as an important energy producer. Its 15.5 billion barrels of proven oil reserves are the second largest in South America. Most of those reserves are located in the Atlantic Ocean, in the Campos and Santos basins off the coasts of Rio de Janeiro and São Paulo states respectively. Carioca-Sugar Loaf, Lula (formerly Tupi), and Jupiter make up the primary fields to be developed in very deep waters. Lula alone is thought to contain between 5 and 8 billion barrels of recoverable reserves. Total estimated potential reserves are greater than 120 billion barrels of reserves for the offshore area.

North Sea

The United Kingdom is an important North Sea producer, and its proven oil reserves of some 3 billion barrels are the largest in the European Union. The supergiant Forties field, identified in 1970, was the second commercial discovery in the North Sea following Norway’s supergiant Ekofisk in 1969. Crude oil production, which peaked in the late 1990s, has declined to less than half of its peak level, however, and Britain, once a net oil exporter, is now a net oil importer.

The broader North Sea, however, is under potential rejuvenation, similar to mature assets elsewhere in the world. The original Ekofisk is expanding again after 40 years of production. In 2011 the operating partners were given approval by Norway to develop and manage Ekofisk South and Eldfisk II, which increases reserves recovery by more than 470 million barrels. Of the five countries with divided interests in the North Sea, Norway holds the most recoverable reserves. In addition, it has the most recent giant field discovery, the Johan Sverdrup, 2010. The field has an estimated 1.7 to 3.3 billion barrels total reserves.